The present invention relates to agricultural tillage implements; and more particularly, to a tillage implement having a vertically adjustable subframe.
Apparatus of this type has, in recent years, become a popular alternative to tillage practices which have existed for decades, such as moldboard plowing. These more recently popularized implements include a main frame provided with support wheels and including a first set of tillage tools. The working depth of these tools is set normally by an hydraulic cylinder operated from the tractor which adjusts the height of the main frame relative to the support wheels. A subframe is carried by the main frame, normally in a forward position; and it includes a second set of tillage implements and an independently operated lift linkage which also includes an hydraulic cylinder for setting the working depth of the tools on the subframe. In this manner, the working depth of both sets of tools may be separately adjusted to the desired level. One such system is disclosed in U.S. Pat. No. 4,245,706 of William J. Dietrich, Sr., issued Jan. 20, 1981 and owned by the assignee of this application.
One of the major problems encountered in the field by a tillage implement is breakage or damage when the machine encounters an obstruction in the field, such as a rock. If a rock is large and near the surface, and the farmer misses seeing it in time, it can actually cause breakage of parts of the implement. In some instances, such as moldboard plows, reset mechanisms, some of which are automatic, are used so that each plow unit is separately equipped with its own protection mechanism. In lighter tillage apparatus, such as chisel plows or cultivators, a spring cushion mechanism may be employed on each separate tool, permitting it to be raised when it encounters an obstruction, and then to be reset under force of the spring after the obstruction is passed.
In the case of implements of the type described having an independently adjustable subframe, the working depth of the tools mounted on the subframe is typically set by a lift mechanism including a double-acting hydraulic cylinder. Once the working depth is set, the lift mechanism becomes rigid. The individual tools carried by the subframe may be provided with their own cushion mechanism, but such mechanisms are limited in the amount of vertical tool displacement they can accomodate and they add to the cost of the machine.
Thus, even where a spring cushion mechanism is provided for each tool, when a large obstacle is encountered, there is an upward force on the subframe which is transmitted through the lift linkage and hydraulic cylinder tending to lift the entire implement. The resulting shock can be damaging, considering the high speed at which lighter tillage apparatus is sometimes pulled, and further considering the tendency to use wider, and therefore heavier, machines to minimize the number of passes required to work a given field. The problem is even further exacerbated in implements where the rear tools are chisel plows or cultivator points because the force resisting vertical displacement of the machine includes not only the entire weight of the machine, but the force required to lift the points from their working positions. That force is truly considerable and damage can easily result. Provision was made for overcoming this problem in the implement described in the application of William J. Dietrich, Sr. for TILLAGE APPARATUS HAVING A VERTICALLY ADJUSTABLE SUBFRAME, Ser. No. 365,783, filed Apr. 5, 1982, now abandoned. In that system, a pressure relief valve is included in the hydraulic cylinder circuit for the subframe lift linkage. If an excessive lifting force is encountered by the subframe, the force is applied to the hydraulic cylinder tending to cause it to extend, which would permit the subframe to raise in response to the presence of the obstacle. This force is translated to a pressure in the rod end of the lift hydraulic cylinder; and if that pressure exceeds a predetermined value, a pressure relief valve is actuated to enable fluid to flow from the rod end of the cylinder to the butt end of the cylinder and permit the cylinder to elongate and the subframe to raise.
The solution to the problem afforded by the pressure relief valve described above is considered a significant advance because it does prevent damage to the implement. However, because fluid flows from the rod end of the cylinder to the butt end in overcoming the obstacle, the effect is to lengthen the cylinder so that the subframe does not reset to its original position. This is not a major problem in the operation of the implement because it simply requires that the operator actuate the hydraulic control valve to reset the subframe to its desired operating depth. Fortunately, farmers are sensitive to the operation of a tillage implement and readily sense when it has encountered a large obstacle; and the small inconvenience of resetting the operating depth of the subframe is more than compensated by the reduction in damage to the implement when a large obstruction is encountered.